Apparatus and methods for stabilization and control of fiber devices and fiber devices including the same

ABSTRACT

A mounting platform provides support and packaging for one or more fiber Bragg gratings and electronic circuitry (e.g., heaters, coolers, piezoelectric strain providers, temperature and strain sensors, feedback circuitry, control loops), which may be printed on or on the mounting platform, embedded in the mounting platform, or may be an “off-board” chip solution (e.g., the electronic circuitry may be attached to the mounting platform, but not formed on or defined on the mounting platform). The fiber Bragg gratings are held in close proximity to the electronic circuitry, which applies local and global temperature and/or strain variations to the fiber Bragg gratings to, for example, stabilize and/or tune spectral properties of the fiber Bragg gratings so that spatial variations in the fiber Bragg gratings that result from processing and manufacturing fluctuations and tolerances can be compensated for.

BACKGROUND

[0001] 1. Field

[0002] Embodiments of the present invention relates to photonic devicesand, more particularly, to stabilization and control of in-fiberphotonic devices.

[0003] 2. Background Information

[0004] An optical transmission system transmits information from oneplace to another by way of a carrier whose frequency is in the visibleor near-infrared region of the electromagnetic spectrum. A carrier withsuch a high frequency is sometimes referred to as an optical signal, anoptical carrier, or a lightwave signal.

[0005] An optical transmission system typically includes several opticalfibers. Each optical fiber includes several channels. A channel is aspecified frequency band of an electromagnetic signal, and is sometimesreferred to as a wavelength. One link of an optical transmission systemtypically has a transmitter, the optical fiber, and a receiver. Thetransmitter converts an electrical signal into the optical signal andlaunches it into the optical fiber. The optical fiber transports theoptical signal to the receiver. The receiver converts the optical signalback into an electrical signal.

[0006] An optical transmission system that transmits more than onechannel over the same optical fiber is sometimes referred to as amultiple channel system. The purpose for using multiple channels in thesame optical fiber is to take advantage of the unprecedented capacityoffered by optical fibers. Essentially, each channel has its ownwavelength, and all wavelengths are separated enough to prevent overlap.

[0007] One way to transmit multiple channels is through wavelengthdivision multiplexing, whereupon several wavelengths are transmitted inthe same optical fiber. Typically, several channels are interleaved by amultiplexer, launched into the optical fiber, and separated by ademultiplexer at a receiver. Along the way, channels may be added ordropped using an add/drop multiplexer or switched using opticalcross-connect switches. Wavelength division demultiplexing elementsseparate the individual wavelengths using frequency-selectivecomponents, which can provide high reflectivity and high wavelengthselectivity with the aim of increasing the transmission capacity ofoptical fibers.

[0008] Many of these frequency-selective components are in-fiberphotonic devices in that the devices are part of an optical fiber.In-fiber devices have a large number of advantages. One advantage isthat coupling of optical signals in and out of the optical fiber toanother discrete photonic device (e.g., discrete filter) is avoided,which allows the optical transmission system to achieve much lowerinsertion losses and to increase long-term device reliability. Anadditional advantage is that polarization effects are reduced becausecylindrical symmetry is maintained.

[0009] One of the limitations of in-fiber devices is that they aredifficult to control using external inputs, to tune or to stabilizedevice properties, for example. For instance, one such in-fiber deviceis a fiber Bragg grating, which can be used as a temporally invariantoptical filter. The physical properties (e.g., strain, temperature) offiber Bragg gratings typically should be stabilized so that thefiltering properties of the gratings are stabilized. When a fiber Bragggrating is attached to a substrate, however, the filtering properties ofthe fiber Bragg grating may be affected by the physical characteristicsof the substrate.

[0010] Morey et al., U.S. Pat. No. 5,042,898 (hereinafter “Morey”),disclose temperature compensated embedded Bragg grating filters in whichtemperature-varying longitudinal strains are configured to compensatecentral wavelength changes attributable to temperature changes. Morey islimited, however, in that it does not compensate for other environmentalconditions to which embedded Bragg grating filters may be subjected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the drawings, like reference numbers generally indicateidentical, functionally similar, and/or structurally equivalentelements. The drawing in which an element first appears is indicated bythe leftmost digit(s) in the reference number, in which:

[0012]FIG. 1 is a schematic diagram of a photonic device according to anembodiment of the present invention;

[0013]FIG. 2 is a schematic diagram of a photonic device according to analternative embodiment of the present invention;

[0014]FIG. 3 is a perspective diagram (side view) of a photonic deviceaccording to an embodiment of the present invention in which thesubstrate is a laminated structure;

[0015]FIG. 4 is a perspective diagram (top view) of the example photonicdevice illustrated in FIG. 3;

[0016]FIG. 5 is a perspective diagram (top view) of a photonic deviceaccording to an alternative embodiment of the present invention;

[0017]FIG. 6 is a perspective diagram (top view) of a photonic deviceaccording to an alternative embodiment of the present invention in whichthe substrate is a material that includes a V-groove;

[0018]FIG. 7 is a perspective diagram (side view) of the examplephotonic device illustrated in FIG. 6;

[0019]FIG. 8 is a perspective diagram (top view) of an optical system (aMichelson interferometer) according to an embodiment of the presentinvention;

[0020]FIG. 9 is a perspective diagram (top view) of an optical system (aMach-Zender interferometer) according to an alternative embodiment ofthe present invention; and

[0021]FIG. 10 is a flowchart illustrating an approach to fabricating aphotonic device according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0022] Embodiments of the present invention are directed to photonicdevices. In the following description, numerous specific details, suchas particular processes, materials, devices, and so forth, are presentedto provide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventioncan be practiced without one or more of the specific details, or withother methods, components, etc. In other instances, well-knownstructures or operations are not shown or described in detail to avoidobscuring understanding of this description.

[0023] Some parts of this description will be presented using terms suchas in-fiber device, strain, photonic device, piezoelectric, laminate,and so forth. These terms are commonly employed by those skilled in theart to convey the substance of their work to others skilled in the art.

[0024] Various operations will be described as multiple discrete blocksperformed in turn in a manner that is most helpful in understanding theinvention. However, the order in which they are described should not beconstrued to imply that these operations are necessarily order dependentor that the operations be performed in the order in which the blocks arepresented.

[0025] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure, process,block, or characteristic described in connection with the embodiment isincluded in at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

[0026]FIG. 1 is a schematic diagram of a photonic device 100 accordingto an embodiment of the present invention. The photonic device 100 maybe fiber-based hybrid photonic device in which one or more appropriatetransducers are situated in proximity to a fiber Bragg grating. Forexample, the photonic device 100 may be an interferometric device basedon combinations of splitters and fiber Bragg gratings (e.g., a filterassembly having a tunable dispersion compensator). Alternatively, thephotonic device 100 may be a switch and/or a fiber-based variableoptical attenuator. Of course, this list is not exhaustive and otherphotonic devices may be implemented as embodiments of the presentinvention.

[0027] The example photonic device 100 includes a substrate 102 and anin-fiber device 104 disposed in or on the substrate 102. The photonicdevice 100 also includes a pair of pigtails 108 and 110, a sensor 112, atransducer 114, a feedback circuit 116, and set of wires 118. In theembodiment shown in FIG. 1, the substrate 102 supports the in-fiberdevice 104, the sensor 112, the transducer 114, the feedback circuit116, and the wires 118. Alternatively, the feedback circuit 116 may be“off-board” circuitry, such as a set of feedback electronics mountedupon a suitable substrate that is not substrate 102. Such “off-board”circuitry may comprise a single electronic chip or multiple electroniccomponents. The sensor 112 may provide input to the feedback circuit116, which may control the transducer 114. The wires 118 may couplecommunication between the feedback circuit 116 and the sensor 112. Thewires 118 also may couple communication between the feedback circuit 116and the transducer 114. The pigtails 108 and 110 may couple opticalinput and output to and from the photonic device 100.

[0028] According to embodiments of the present invention, the substrate102 may be a laminated structure as described with respect to FIGS. 3and 4, a material containing a v-groove as described with respect toFIG. 5, or a two-part composite structure that has a v-groove asdescribed with respect to FIGS. 6 and 7. The substrate 102 may be asemiconductor substrate such as silicon. Of course other embodiments arepossible for the substrate 102, such as a “flip-chip” platform.Alternatively, the substrate 102 may be a material suitable to functionas a piezoelectric transducer.

[0029] The in-fiber device 104 may be a fiber Bragg grating. For ease ofexplanation embodiments of the present invention may be described withreference to the in-fiber device 104 being a fiber Bragg grating. Ofcourse, other in-fiber devices may be implemented as embodiments of thepresent invention. Such devices might include optical splitters, opticalcouplers, optical switches, variable optical attenuators, opticalinterleavers, etc.

[0030] The pigtails 108 and 110 may be optical fibers. The sensor 112may be a strain sensor, a thermocouple, or a thermistor, which sensestrain and/or temperature. Of course, the sensor 112 may be anothersuitable sensor. The transducer 114 may be a heating element or acooling element, which heat and/or cool the in-fiber device, substrate,or other component(s) of the photonic device 100. Suitable heatingelements may be one or more off-the-shelf heaters, such as one or moreNiChrome wires or other suitable heating elements. Suitable coolingelements may be one or more Peltier heater/coolers. Of course, thetransducer 114 may be another suitable transducer, such as apiezoelectric transducer.

[0031] In one embodiment of the present invention, the fiber Bragggrating 104 has a particular spectral response. Placement of one or moretransducers, sensors, and/or control loops (e.g., temperature sensorsand heating/cooling elements) near the fiber Bragg grating facilitatesclosed-loop control of temperature and control of fiber Bragg gratingspectral properties and permits spatially-varying temperatures andstrains so that spatial variations in a fiber Bragg grating that resultfrom processing and manufacturing fluctuations and tolerances can becompensated for.

[0032] Alternatively, such control permits micro-tuning of the shape andposition of the fiber Bragg grating spectrum. In the representativeexamples discussed below, thermal control is used, although other formsof control, such as piezoelectric strain control can also be used. Inaddition, the mounting platforms and associated electronics andtransducers are suitable for the mounting and control of otherfiber-based devices and not just fiber Bragg gratings. Such devicesinclude splitters, combiners, variable optical attenuators, opticalswitches, etc.

[0033]FIG. 2 is a schematic diagram of a photonic device 200 accordingto an alternative embodiment of the present invention. The photonicdevice 200 is similar to the photonic device 100 in that the photonicdevice 200 includes a substrate 202, which supports an in-fiber device204 and includes and a pair of pigtails 208 and 210. The photonic device200 also includes a pair of sensors 212 and 214, a pair of transducers216 and 218, a pair of feedback circuits 220 and 222, and two sets ofwires 224 and 226. The two sets of transducers, sensors, and feedbackcircuits may be used to separately control to different portions ofin-fiber device 204.

[0034] Although only two sets of sensors 212 and 214, transducers 216and 218, feedback circuits 220 and 222, and wires 224 and 226 are shownin FIG. 2, a person of ordinary skill in the relevant art(s) willreadily recognize that other embodiments of the present invention mayinclude more than two sets. Additionally, although individual feedbackcircuits are depicted for clarity, a person of ordinary skill in therelevant art(s) will readily recognize that a single feedback circuitmay be used to control multiple transducer/sensor pairs. A person ofordinary skill in the relevant art(s) will readily recognize that thenumber of sensors does not have to equal the number of transducers.

[0035] In order to manufacture the example photonic device 100 or 200according to embodiments of the present invention, the designerconfigures the characteristics of the various components to based on anintended fiber Bragg grating 104 or 204 spectrum according to a centralwavelength (typically dependent on global changes in the fiber Bragggrating 104 or 204 environment), as well as the spectral shape of thefilter function (typically dependent on local changes in the fiber Bragggrating 104 or 204 environment). Temperature and strain distributionscan be selected to achieve an intended spectrum. For example, strainsproduce by changes in the size or shape of the device due to temperaturefluctuations, humidity changes, aging, and electric field inducedeffects (piezoelectric), can be compensated for, or selected. Sucheffects can be compensated for or used to achieve an intended spectrum.For example, an enhanced tuning range can be achieved if a device isconfigured so that small changes in temperature produce large changes instrain and consequently large spectral shifts.

[0036] In the manufacture of the example photonic devices 100 or 200according to embodiments of the present invention, manufacturingimperfections that might result in large temperature fluctuations alongthe length of the fiber Bragg grating 104 or 204 can be avoided. Suchfluctuations can result from a distribution of thermal impedancesbetween different heaters or heater regions and the fiber Bragg grating104, and/or a distribution of thermal impedances between differenttemperature sensors and the fiber Bragg grating 104 or 204. Adistribution of thermal impedances can also result form angularmisalignment between the electronic elements (e.g., feedback circuitry,wires) and the fiber Bragg grating 104 or 204 or material variationssuch as thickness, density, grain structure, defects, etc. Additionally,spatial variations within electronic elements as well as piece-to-piecevariations between the electronic elements, such as resistance, thermaltransfer rates, thermal sensitivity, etc., can be calibrated.

[0037] The separation of the fiber Bragg gratings can be determinedbased on an intended temperature difference between fiber Bragg gratingsand the thermal conductivity of the substrate. Manufacturing of multiplecontrolled fiber Bragg gratings in individual substrates could beimproved by starting with a platform containing multiple fiber Bragggratings and dicing into individual components.

[0038] In one embodiment of the present invention, a very uniformtemperature may be provided (e.g., for use in filtering applications).This can be achieved by using a substrate with a high thermalconductivity. Alternatively, a large number of calibratedheater/temperature sensor pairs can be individually controlled toaccount for material and assembly fluctuations.

[0039] Note that the properties of all of the components of photonicdevices implemented in accordance with embodiments of the presentinvention may be maintained consistent and controlled over and extendedrange of operational environments for the expected lifetime of thephotonic devices. Therefore, the thermal expansion coefficients and thesusceptibility to changes in humidity, electric field, magnetic field,or any other environmental variable may be maintained consistent overtime and along the length of the fiber Bragg grating 104 or 204.

[0040]FIG. 3 is a side view of a photonic device 300 according toembodiments of the present invention in which the substrate is alaminated structure. The photonic device 300 includes control circuitry302 (comprising, for example, one or more transducers, sensors, feedbackcircuitry) and an in-fiber device 304, which are mounted in/on alaminated mounting structure 306. The laminated mounting structure 306includes an upper substrate 308, a lower substrate 310, and an adhesivelayer 312. The upper substrate 308 includes an outer surface 307 and aninner surface 309. In one embodiment of the present invention, thein-fiber device 304 is attached to the laminated mounting structure 306,using an adhesive such as a two-part epoxy, an ultraviolet (UV) curableepoxy, solder, or other suitable attachment techniques, for example.According to alternative embodiments of the present invention, alignmentmarks 320 or other indicators may be provided on the laminated mountingstructure 306 to aid in positioning the in-fiber device 304 with respectto the control circuitry 302.

[0041] In the embodiment of the present invention shown in FIG. 3, thelaminated mounting structure 306 includes a single in-fiber device 304.However, in alternative embodiments of the present invention, thelaminated mounting structure 306 includes more than one in-fiber device.

[0042] The upper substrate 308 may be fabricated from polymers, glasses,crystal, insulators, piezoelectric material, or other suitable material.The lower substrate 310 also may be fabricated from polymers, glasses,crystal, insulators, piezoelectric material, or other suitable material.The material of the lower substrate 310 need not be same material as theupper substrate 308.

[0043] The adhesive layer 312 holds upper substrate 308 and lowersubstrate 310 together. The adhesive layer 312 also holds the in-fiberdevice 304 in place. According to an embodiment of the presentinvention, the adhesive layer may be an elastomeric material or othersuitable material.

[0044] The control circuitry 302 may be printed or mounted on the outersurface 307 of the upper substrate 308, may be formed in the innersurface 309 of the upper substrate 308 or may be embedded directly inthe laminated mounting structure 306. Alternatively, the controlcircuitry 302 may be within the adhesive layer 303 or may incorporate an“off-board” chip solution (e.g., some portion of the control circuitry302 may not be attached to the laminated mounting structure 306.

[0045] Strain distribution strains along the length of the fiber Bragggrating 304 can also be characterized and controlled according toembodiments of the present invention. Different regions of the fiberBragg grating 304 are in physical contact with different regions of theadhesive layer 312. In one embodiment, reduction of strain differentialsis promoted by having the adhesive 312 have properties that do not varyfrom point to point within the laminate mounting structure 306.Alternatively, if the adhesive 312 is elastomeric, it can serve totransfer the structural properties of the outer laminate materials ofthe substrates 308 and 310 to the fiber Bragg grating 304. In this case,the properties of the outer laminate materials of the substrates 308 and310 may be consistent over a length scale comparable to the length ofthe fiber Bragg grating 304.

[0046]FIG. 4 is a top view of the photonic device 300 according toembodiments of the present invention showing the control circuitry 302in more detail. Also shown in FIG. 4 are input and output pigtails 402.As illustrated in FIG. 4, the control circuitry 302 may include heatingelements 404, temperature sensors 406, and feedback circuitry 408. Insome embodiments, heating elements 404 could be replaced with straintransducers or other transducers known in the art. Similarly,temperature sensors 406 could be replaced with strain sensors or othersensors known in the art. Optional input 420 may be included to provideset point information for the feedback circuitry.

[0047]FIG. 5 is a perspective diagram (top view) of a photonic device500 according to an alternative embodiment of the present invention inwhich a substrate 504 includes a V-groove 506. A fiber Bragg grating 502is mounted on the substrate 504 inside the V-groove 506. The fiber Bragggrating 504 includes an active region 508 and inactive regions 509. Thephotonic device 500 also includes heaters 510, temperature sensingelements 512, and feedback circuits 514. The fiber Bragg grating 502 ismounted at two points 520 and 522, which are outside the active region508. Bonding of the fiber Bragg grating 502 to the substrate 504 can beachieved by applying adhesive at the two points 520 and 522 such thatthe adhesive surrounds but does not contact the active region 508. Thefiber Bragg grating 502 can be mounted with a controlled amount of“pre-strain” or no strain depending upon the application requirements.

[0048] The V-groove 506 serves to locate the fiber Bragg grating 502with respect to the substrate 504 and may be a “v-groove” that iscommonly used for locating a strand of optical fiber with respect to asubstrate.

[0049] In the embodiment of the present invention illustrated in FIG. 5,the heaters 510, temperature sensing elements 512, and feedback circuits514 are printed upon the substrate 504 in close proximity to theV-groove 506. Alternatively, if the substrate 504 is piezoelectric, theheaters 510 may be replaced with electrodes and the temperature sensingelements 512 may be replaced with strain sensors.

[0050] Note that the circuitry (e.g., the heaters 510, temperaturesensing elements 512, and feedback circuits 514 in FIG. 5, or theelectrodes and strain sensors in alternative embodiments) may be printedon the “top” surface of the substrate 504 next to the V-groove 506, onthe “bottom” surface of the substrate 504 in substantial alignment withthe groove 506, or within the V-groove 506.

[0051] Note further that the dimensional stability of substrate 504 mayaffect the spectrum of fiber Bragg grating 502 through strainvariations. The dimensions of substrate 504 could be affected throughchanges in temperature, electric field, humidity, magnetic field, etc.In the embodiment in which the fiber Bragg grating 502 is mounted atpoints 520 and 522, the strain variations along the length of the fiberBragg grating 502 may be very small, allowing strain to be considered asa global variable. Temperature variations along the length of the fiberBragg grating 502 may be controlled as well.

[0052]FIG. 6 is a perspective diagram (top view) of an unassembledphotonic device 600 according to an alternative embodiment of thepresent invention, in which a “flipchip” platform is used and onesubstrate is used to hold an in-fiber device and a second substrate isused to hold control circuitry. For example, FIG. 6 shows a fiber Bragggrating 602 mounted on a substrate 604 inside a groove 606. The fiberBragg grating 602 includes an active region 608. The fiber Bragg grating602 is mounted at two points 620 and 622, which are outside the activeregion 608. Bonding of the fiber Bragg grating 602 to the substrate 604can be achieved by applying adhesive at the two points 620 and 622 suchthat the adhesive surrounds but does not contact the active region 608.FIG. 6 also shows control circuitry 609, comprising one or more sensors610, one or more transducers 612, and one or more feedback circuits 614,mounted, printed, or embedded on a substrate 616. According to theembodiment illustrated in FIG. 6, sensors 610 are temperature sensorsand transducers 612 are heaters. Of course, one or more of the sensors610 could be replaced with a different type of sensor and one or more ofthe heaters 612 could be replaced with a different type of heater inalternative embodiments.

[0053] The “flip-chip” design of the photonic device 600 allows for theseparate optimization of the two substrates 604 and 616. For example,the substrate 616 can be optimized for the control circuitry 609 and thesubstrate 604 can be optimized for mounting and controlling the fiberBragg grating 602, with attention being paid to thermal conductivity,thermal expansion coefficient, piezoelectric coefficients, sensitivityto moisture, bonding characteristics, etc. In an embodiment of thepresent invention, the substrate 616 may be silicon and the substrate604 may be glass, which has properties very similarly to the opticalfiber. Alternatively, the substrate 604 may be a more exotic materialwith a tailored thermal expansion coefficient to decrease (e.g.,minimize) or possibly increase, (e.g., maximize) the temperaturedependent spectral shift of the photonic device 600.

[0054]FIG. 7 is a perspective diagram (side view) of the photonic device600 after the two substrates 604 and 616 are put together. For example,the photonic device 600 includes techniques 702 to hold the substrate404 and the substrate 616 together. In one embodiment, the techniques702 includes and adhesive layer and the substrates 604 and 616 arecontacted and bonded such that the fiber Bragg grating 402 and thetransducer circuitry (heaters 610) are substantially aligned. In otherembodiments, other techniques, such as external compression or othersuitable techniques are used to hold the substrates 604 and 616together.

[0055] In one embodiment, the face of the substrate 604 is contacted tothe face of the substrate 616 such that the fiber Bragg grating 602 isin very close proximity to the heating elements 610 and temperaturesensing elements 612. In another embodiment, the groove 606 is designedsuch that top of the fiber Bragg grating 602 is flush with the topsurface of the substrate 604, ensuring good contact with substrate 616without introducing a large amount of transverse compression. In oneembodiment, the adhesive used to bond the fiber Bragg grating tosubstrate 604 in regions 620 and 622 does not form a tall meniscus thatwould interfere with the contacting between substrates 604 and 616.Alternatively, small divots could be placed in the regions of substrate616 that correspond to regions 620 and 622 to allow clearance for theadhesive. These divots could also be used as alignment aids in assemblyof the photonic device 600.

[0056] In some embodiments of the present invention, uniform temperatureand strain variations may be applied to a fiber Bragg grating usingtransducers, sensors, feedback circuitry, and/or other control circuitryto produce a shift of a central wavelength of filter embodying a fiberBragg grating while leaving the shape of the filter unchanged. In otherembodiments, non-position-dependent temperature and strain variationsmay be applied to a fiber Bragg grating.

[0057] In still other embodiments of the present invention,position-dependent temperature and strain variations may be applied to afiber Bragg grating. Such variations can produce local changes ingrating pitch and thereby lead to spectral distortion. In general,requirements on temperature and strain uniformity are functions of thespectral resolution and other spectral properties of the fiber Bragggrating. For example, a filter with a reflective bandwidth of 100picometers (pm) and a temperature coefficient of 10 pm/° C. should notbe subjected to a temperature variation of more than about 0.1° C. toavoid significant spectral distortion. Similarly, the same filter with astrain coefficient of 1 pm/microstrain should not be subjected to astrain variation of more than about 1 microstrain to avoid significantspectral distortion. Note that the 1 pm deviation in central frequencyalong the length of the fiber grating is an order of magnitude estimate.In general, the size of the allowable deviation will depend in detail onthe exact spatial profile of the grating, the effective spatialfrequencies of the strain or temperature deviations, and the devicetolerances.

[0058] In one embodiment of the present invention, more complex methodsof control use temperature and/or strain variations along the length ofthe fiber Bragg grating to configure the spectral shape of the filter.Such variations can be used to control amplitude and phasecharacteristics of the spectral response. Such embodiments may be used atunable dispersion compensators, which control of both the dispersionand the dispersion slope.

[0059] Control of spectral shape using temperature and/or strainvariations may be considered based on two conditions. In a firstcondition, individual control elements and control loops are used tomodify the characteristics of individual subsections of the fiber Bragggrating such that the overall spectral response, which is a combinationof the spectral responses of each of the subsections, is controlled toachieve an intended spectral response. In a second condition, multipleoverlapping control elements are controlled such that a superposition ofthe temperatures/strain variations produces intended spectral responses.Control elements may have associated temperature/strain gradients that“spillover” into adjacent regions. For thermal control based on heatingelements or cooling elements, spillover rates are a function of thermalconductivity and spillover can limit the spatial resolution with whichtemperature variations can be applied to the filter.

[0060]FIG. 8 is a perspective diagram (top view) of an optical system800 according to an embodiment of the present invention, in which twofiber Bragg gratings 802 and 804 are coupled through a splitter/coupler810 to two ports 812 and 814 in a Michelson interferometer configurationand disposed (e.g., embedded) in or on a fiber control platform 816.Four control elements 820, 824, 826, and 828 also are embedded in thefiber control platform 816.

[0061] The splitter/coupler 810 may be any suitable fiber-based couplerand/or splitter, such as a fused fiber coupler. The ports 812 and 814may be fiber pigtails.

[0062] The control elements 820, 824, 826, and 828 each may be one ormore heating elements, one or more temperature sensors, and/or one ormore electronic feedback circuits. The control elements 820, 826, andmay control the spectral characteristics of each fiber Bragg grating 802and 804, and the optical delay in each leg of the Michelsoninterferometer. Control of the optical path difference in the two legsof the Michelson interferometer is achieved through the control elements824 and 828, which modify the length and/or index of refraction of fibersegments 806 and 808, respectively.

[0063] Note that only one of the control elements 824, 828 may beprovided in many applications. However, because heaters are unipolar,having the ability to differentially heat both legs of the Michelsoninterferometer may enhance the resistance of the device to environmentalfluctuations. When properly controlled, the accumulated phase differencefor a given reflected color at splitter/coupler 810 causes the combinedsignal to exit through the output port 814. Mismatches from the idealsituation (in which both total optical paths are exactly matched) willlead to light leaking out through the input port 812.

[0064] In operation, light that enters the optical system 800 throughthe input port 812 that is resonant with fiber Bragg gratings 802 and804 is output through the output port 814 and no light is reflected backthrough input port 812. Light that enters the photonic device 800through input port 812 is split into two equal portions by thesplitter/coupler 810. The light travels along fiber segments 806 and 808and is filtered by the fiber Bragg gratings 802 and 804. Thereflectivity spectra of fiber Bragg gratings 802 and 804 (amplitude andphase) are controlled by control elements 820 and 826, respectively. Thereflected light travels along fiber segments 806 and 808 and isrecombined at splitter/coupler 810.

[0065] Traditional use of fiber Bragg gratings in reflection geometry isachieved through the use of a circulator or a 3 dB splitter. Thecirculator is an expensive free space device with a typical insertionloss of 1.5-2 dB. 3 dB splitters can be manufactured cheaply asall-fiber components, but their use in this geometry introduces a 6 dBloss in the output port and also has a large reflection at the inputport.

[0066] According to embodiments of the present invention, by using thefiber Bragg gratings 802 and 804 in a Michelson interferometer, it ispossible to achieve low insertion loss and low back-reflection at theinput port using an all fiber device. Also, mounting (e.g., embedding)the Michelson interferometer on the fiber control platform 816 allowsfor individual control of the components that make up theinterferometer, which enables a higher performance photonic device.

[0067] In one embodiment of the present invention, the spectral responseof the optical system 800 may be flat over the data bandwidth and overany required amount of spectral drift of the input signal that theoptical system 800 may tolerate. In this embodiment, the amplitude andthe phase of the reflected light may be held at a constant fixedrelationship between the two arms of the interferometer over theoperational bandwidth for the optical system 800. Therefore, therelative temporal delay between the two legs may be constant as afunction of frequency, and the amplitude and group delay characteristicsas a function of frequency of the two fiber Bragg gratings 802 and 804may be matched. Mismatch of the two legs of the interferometer may leadto an increased insertion loss and an increased reflected intensity atthe input port 812.

[0068] In an alternative embodiment, an ideal spectral filter for use inthe optical system 800 may have a flat reflectivity spectrum and aconstant group delay as a function of wavelength, i.e., a perfectmirror. In this embodiment, the two fiber Bragg gratings 802 and 804 mayundergo spectral drift with respect to each other without affecting theoutput of the Michelson interferometer as long as the reflectivityspectra of the two fiber Bragg gratings 802 and 804 both included thespectrum of the input signal.

[0069] Most fiber Bragg gratings have flat spectral responses, but notflat group delay as a function of wavelength. In fact, most fiber Bragggratings (such as those with standard Blackman apodization profiles)have a group delay that increases approximately quadratically withdetuning from the center of the reflection band. For an interferometerutilizing this type of filter, spectral detuning of the filters(relative to each other) will cause delay differences as a function ofwavelength, which swill cause a frequency dependent output and spectralshaping of the output signal. The control for this interferometerdepends upon both the amplitude and phase of the spectral response ofthe fiber Bragg gratings.

[0070] In one embodiment, the sensitivity of the Michelsoninterferometer may be controlled (e.g., minimized) using fiber Bragggratings with tailored group delay profiles, such as those described inUniversity of Southampton (Proceedings of the Optical Fiber Conference2001, Paper MC1). For example, the fiber Bragg gratings 802 and 804 mayhave group delay variations of a few picoseconds (relative to 10 s ofpicoseconds for a standard fiber Bragg grating such as a Blackmanapodized fiber Bragg grating) across the passband, which cansubstantially improve the characteristics of the Michelsoninterferometer, including the insertion loss, return loss and spectralsensitivity.

[0071]FIG. 9 is a perspective diagram (top view) of an optical system900 according to an embodiment of the present invention. In oneembodiment, the optical system 900 is an optical add-drop multiplexer(OADM). Unlike prior art OADM devices that can include a pair of matchedfiber Bragg gratings and two circulators and which can be relativelyexpensive and suffer from optical losses, an all-fiber OADM such as thatembodied in the optical system 900 can include two fiber Bragg gratingsconfigured in a Mach-Zehnder interferometer. By disposing (e.g.,embedding) a fiber-Bragg grating-based Mach-Zehnder interferometer in oron a fiber control platform, the individual components of theMach-Zehnder interferometer can be controlled such that the opticalsystem 900 is stable, has low insertion loss, and offers high isolation.

[0072] The example optical system 900 includes two fiber Bragg gratings902 and 904 in two fibers that have two fiber segments each (906 and907, 908 and 909, respectively), which are coupled through twosplifters/couplers 910 and 911 four ports 912, 913, 914, and 915 in aMach-Zehnder interferometer configuration in or on a fiber controlplatform 916. Six control elements 920, 922, 924, 926, 928, and 930 alsoare mounted in or on the fiber control platform 916.

[0073] The splitters/couplers 910 and 911 may be any suitablefiber-based couplers and/or splitters, such as a fused fiber couplers.The ports 912, 913, 914, and 915 may be fiber pigtails. The controlelements 920, 922, 924, 926, 928, and 930 may be each may be one or moreheating elements, one or more temperature sensors, one or moreelectronic feedback circuits, and/or one or more other control elements.For ease of explanation, the port 912 is defined as the input port, theport 913 is defined as the output port, the port 914 is defined as the“drop” port, and the port 915 is defined as the “add” port for the OADM.

[0074] In operation, light that enters the optical system 900 at theinput port 912 is split into two equal portions by the splitter/combiner911 and travels along the fiber segments 907 and 909 to the fiber Bragggratings 902 and 904. Light that is resonant with the fiber Bragggratings 902 and 904 is reflected back along fiber segments 907 and 909to the splitter/combiner 911 where it is recombined. If the relativedelays along the interferometer arms match, then the reflected lightwill combine coherently and exit through the drop port 914, similar tothe photonic device 800 described above. The control elements 920 and922 may be used to control the delays induced by fiber segments 907 and909 and the control elements 926 and 928 may be used to control thespectral responses of the fiber Bragg gratings 902 and 904. Thuswavelengths that are resonant with the fiber Bragg gratings 902 and 904that are incident at input port 912 are dropped at the drop port 914.

[0075] Light that is not resonant with the fiber Bragg gratings 902 and904 passes through the fiber segments 908 and 906 to be recombined atthe splitter/combiner 910. The optical lengths of fiber segments 908 and906 are controlled such that the light coherently recombines at outputport 913. As this is a symmetric device, light that is resonant with thefiber Bragg gratings 902 and 904 that is incident at add port 915 willbe output at output port 913, along with light that was incident at theinput port 912 that passed through the device to output port 913.

[0076] The tolerances on the optical system 900 are similar to those ofthe photonic device 800 described above. Optical path mismatches, dueeither to mismatches between fiber segment pair 907 and 909, fibersegment pair 906 and 908, or the fiber Bragg gratings 902 and 904 maylead to a performance degradation, as evidenced by an increasedinsertion loss, and/or an increased cross talk between supposedlyisolated ports, such as increased output at the drop port 914 fornon-resonant signals incident at input port 912, increased output at theoutput port 913 for resonant signals incident at the input port 912,increased output of any type at the input port 912, increased output atthe drop port 914 of resonant signals incident at the add port 915, etc.Use of fiber Bragg gratings with tailored group-delay profiles enhancesthe performance of the optical system 900.

[0077] Embodiments of the invention can be implemented using hardware,software, or a combination of hardware and software. In implementationsusing software, the software may be stored on a computer program product(such as an optical disk, a magnetic disk, a floppy disk, etc.) or aprogram storage device (such as an optical disk drive, a magnetic diskdrive, a floppy disk drive, etc.).

[0078] The above description of illustrated embodiments of the inventionis not intended to be exhaustive or to limit the invention to theprecise forms disclosed. While specific embodiments of, and examplesfor, the invention are described herein for illustrative purposes,various equivalent modifications and alterations are within the spiritand scope of the appended claims, as those skilled in the relevant artwill recognize. These modifications can be made to the invention inlight of the above detailed description.

[0079] The terms used in the following claims should not be construed tolimit the invention to the specific embodiments disclosed in thespecification and the claims. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. An apparatus, comprising: a substrate; at leastone in-fiber device disposed in or on the substrate; at least one sensoroperationally coupled to the in-fiber device; at least one transduceroperationally coupled to the in-fiber device; and feedback circuitryoperationally coupled to the sensor and the transducer.
 2. The apparatusof claim 1, wherein the in-fiber device is a fiber Bragg grating.
 3. Theapparatus of claim 1, wherein the substrate is a laminated mountingstructure.
 4. The apparatus of claim 1, wherein the substrate includesgroove, at least a portion of the in-fiber device being disposed in thegroove.
 5. The apparatus of claim 1, wherein the substrate is aflip-chip platform.
 6. The apparatus of claim 3, wherein at least one ofthe sensor or transducer is embedded in the laminated mountingstructure.
 7. The apparatus of claim 1, wherein the substrate isfabricated from at least one of a polymer, glass, crystal, insulator, orpiezoelectric material.
 8. The apparatus of claim 7, wherein thelaminated mounting platform includes indicators to aid in aligning thein-fiber device and the electronic circuitry.
 9. The apparatus of claim4, wherein an active region of the in-fiber device is disposed in thegroove and an inactive region of the in-fiber device is attached to thesubstrate using at least one of a two-part epoxy, an ultraviolet (UV)curable epoxy, or solder.
 10. The apparatus of claim 4, wherein at leastone of the sensor or transducer is in substantial alignment with thegroove.
 11. The apparatus of claim 5, wherein the flip-chip platformcomprises a first flip-chip substrate, a second flip-chip substrate, andan adhesive between the first and second flip-chip substrates to holdthe first flip-chip and second flip-chip substrates together.
 12. Theapparatus of claim 11, wherein at least one of the sensor or transduceris printed on the first flip-chip substrate.
 13. The apparatus of claim12, wherein a second flip-chip substrate includes alignment marks toalign at least one of the sensor, transducer, or in-fiber device witheach other.
 14. A method of fabricating a photonic device, comprising:disposing at least one in-fiber device in or on a substrate;operationally coupling at least one sensor to the in-fiber device;operationally coupling at least one transducer to the in-fiber device;and operationally coupling feedback circuitry to the sensor and thetransducer.
 15. The method of claim 14, wherein disposing at least onein-fiber device in or on a substrate comprises disposing at least onefiber Bragg grating in or on a substrate.
 16. The method of claim 14,wherein disposing at least one in-fiber device in or on a substratecomprises disposing at least one fiber Bragg grating in or on alaminated mounting structure.
 17. The method of claim 14, whereindisposing at least one in-fiber device disposed in or on a substratecomprises disposing at least one in-fiber device in or on a flip-chipplatform.
 18. The method of claim 14, wherein disposing at least onein-fiber device disposed in or on a substrate comprises disposing atleast one in-fiber device in or on a grooved substrate.
 19. The methodof claim 15, wherein operationally coupling at least one sensor to thefiber Bragg grating comprises operationally coupling at least one strainsensor or at least one temperature sensor to the fiber Bragg grating.20. The method of claim 15, wherein operationally coupling at least onetransducer to the fiber Bragg grating comprises operationally couplingat least one of a heating element, cooling element, or a piezoelectrictransducer to the fiber Bragg grating.
 21. A system, comprising: aninput port and an output port disposed on a substrate; asplitter/coupler disposed on the substrate and operationally coupled tothe input port and output port; a first leg of a fiber Bragggrating-based Michelson interferometer disposed on the substrate andcoupled to the splitter/coupler, a first set of control elementsoperationally coupled to the first leg; and a second leg of the fiberBragg grating-based Michelson interferometer disposed on the substrateand coupled to the splitter/coupler, second set of control elementsoperationally coupled to the second leg.
 22. The system of claim 21,wherein the first or second sets of control elements comprises circuitryto control spectral characteristics of the first or second legs,respectively.
 23. The system of claim 21, wherein the first and secondsets of control elements comprises circuitry to control optical delaybetween the first leg and the second leg.
 24. The system of claim 22,wherein the first or second sets of control elements comprises at leastone of a sensor, transducer, or feedback circuitry.
 25. The system ofclaim 22, wherein the first or second sets of control elements comprisesat least one of a sensor, transducer, or feedback circuitry.
 26. Asystem, comprising: an input port, an add port, a drop port, and anoutput port each disposed on a substrate; a first splitter/couplerdisposed on the substrate and operationally coupled to the input portand drop port; a second splitter/coupler disposed on the substrate andoperationally coupled to the add port and output port a first leg and asecond leg of a fiber Bragg grating-based Mach-Zehnder interferometerdisposed on the substrate and coupled between the first and secondsplitter/couplers; and a first set and a second set of control elementsoperationally coupled to the first and second legs, respectively. 27.The system of claim 26, wherein the first or second sets of controlelements comprises circuitry to control spectral characteristics of thefirst or second legs, respectively.
 28. The system of claim 26, whereinthe first and second sets of control elements comprises circuitry tocontrol optical delay between the first leg and the second leg.
 29. Thesystem of claim 27, wherein the first or second sets of control elementscomprises at least one of a sensor, transducer, or feedback circuitry.30. The system of claim 28, wherein the first or second sets of controlelements comprises at least one of a sensor, transducer, or feedbackcircuitry.